The present invention relates to television demodulating system and, more particularly, to a circuit and method for reducing differential phase errors associated with receivers designed for reception of stereo sound.
Most contemporary television receivers utilize some means of regenerating the video IF carrier so that the IF signal can be demodulated. Any change in the phase of this regenerated IF carrier with signal amplitude will result in a loss of performance in both the video and sound channels. The phase locked loop (PLL) and pseudo synchronous demodulating systems are the two most commonly used methods of regenerating the video carrier. Both methods incorporate a limiter and an IF bandpass filter. The latter precedes the IF amplifier and demodulators and provides most of the selectivity of the TV receiver. Unfortunately, due to the nature of the TV signal having unequal side bands (vestigial transmission) and the action of the IF filter, quadrature differential phase errors are inherent to the system and cause problems with the above demodulating system. In a PLL demodulator this produces a signal at the output of the phase detector proportional to the video which will in turn phase modulate the VCO of the PLL. The end result of this will be changes in color with video amplitude, notably high frequency video beat notes between the sound and video signals and audible video on sound. With the advent of stereo TV sound, the latter interference of the FM sound intercarrier by the video signal has become very important. An acceptable fifty to sixty decibels of sound to interference for a mono-sound transmission being degraded to an unacceptable twenty to thirty decibels for stereo transmission.
Similarly, differential phase changes may occur due to changes in phase with signal amplitude through the limiter stage of the demodulating system. This change in phase also gives rise to a change in the output current of the phase detector. However, the change is in-phase as opposed to the quadrature change produced by the IF filter as described above.
Both of the above described components of the differential phase are highly undesirable. Some prior art stereo televisions using PLL type demodulating systems attempt to minimize differential phase effects by using a separate sound IF demodulating system that duplicates the video demodulating system. This approach requires a separate sound IF filter and adds to the cost of manufacture of the television receiver. Moreover, this approach does not eliminate the in-phase component of the differential phase effect due to the limiter stage that is still present in the separate sound IF section.
Hence, a need exists to reduce differential phase effects in televisions and, in particular, those designed for stereo sound reception.